This is mine first build but i hope u'll enjoy that! ;)

Pros:
-when u learn how to play u'll enjoy her :)
-passive allows u to spam skills
-ur Q is almost as hate as zilean bombs
-poisons!!!
-ur Q and W are great skills to escape from enemies
-ur Q allowss u to check bushes (if u hit someone u'll get speed buff)
-almost noone know that champ good becouse its not so many cassiopeias :(
-petrificate is cool skill when used on team fight well all their team is dead and ur team is probably with more than 1/2 hp

Cons:
-weak. she got less hp than lux
-u have to imagine where ur enemy will go and use here ur spells its rather hard
-ulti get so small range u have to make that fast cuz enemies have to face on you

i pick up heal becouse its not so easy to do ur ulti and u'll get many hits but after all u'll get a moment to use ulti. Flash is a great skill when u chase enemy u can just flash right before him and use petrificate to kill him easy :)

u should spam so much ur Q and use W just to slow enemies that they could not run away if u get ulti ready u should use that to just petrificate 2 or more enemies just to get less dmg to ur team

1.when u want to kill enemy in 1v1 u should :

Q->E->W->E->Q etc

when u got petrificate and enemy is runing to u use that first if not do not waste that to just slow enemy

2. when u see enemy is runing out just use ur Q on creeps or champion (if he is closer to u just use W and u'll get him) rest is basic combo
The alternative version of chasing enemy is that what i wrote before + flash on front of him and use ur ulti whatever u'll do u'll kill him :)

3. When target is poisoned ur E Cooldown is reduced to 0.5s that mean u should poison target and spam with ur E :]

its simply ur Q get nice range so u can just spam that skill on ur enemy if he is runing out of ur skill just use Q on minions(rather magic ones cuz there is a little chance that enemy will stop over here) When u destroy enemy turret u should change lane to help others (maybe few of u wont agree with me but i think when its 2 v 3 they should react that one from ur lane before will go where u go)